Data and Computer Communications

Data Transmission

Data Transmission

What we've got here is failure to communicate.

Paul Newman in

Cool Hand Luke

Data Transmission

• quality of the signal being transmitted The successful transmission of data depends on two factors: • characteristics of the transmission medium

Transmission Terminology Data transmission occurs between transmitter and receiver over some transmission medium.

Communication is in the form of electromagnetic waves.

Guided media

twisted pair, coaxial cable, optical fiber

Unguided media (wireless)

air, vacuum, seawater

Transmission Terminology

Transmission Terminology



Simplex

 signals transmitted in one direction • eg. Television 

Half duplex

 both stations transmit, but only one at a time • eg. police radio 

Full duplex

 simultaneous transmissions • eg. telephone

Frequency, Spectrum and Bandwidth Time Domain Concepts



analog signal

• 

digital signal

• signal intensity varies smoothly with no breaks signal intensity maintains a constant level and then abruptly changes to another level 

periodic signal

• signal pattern repeats over time 

aperiodic signal

• pattern not repeated over time

Analog and Digital Signals

Periodic Signals

Sine Wave

(periodic continuous signal) 

peak amplitude (A)

 maximum strength of signal  typically measured in volts 

frequency (f)

  rate at which the signal repeats Hertz (Hz) or cycles per second    T = 1/f

phase (



)

 period (T) is the amount of time for one repetition relative position in time within a single period of signal

Varying Sine Waves s(t) = A sin(2



ft +



)

Wavelength (



) the wavelength of a signal is the distance occupied by a single cycle can also be stated as the distance between two points of corresponding phase of two consecutive cycles assuming signal velocity v, then the wavelength is related to the period as



= vT

•

especially when v=c c = 3*108 ms-1 (speed of light in free space)



or equivalently f = v

Frequency Domain Concepts

 signals are made up of many frequencies  components are sine waves  Fourier analysis can show that any signal is made up of components at various frequencies, in which each component is a sinusoid  can plot frequency domain functions

Addition of Frequency Components (T=1/f)

c is sum of

f & 3f

Frequency Domain Representations

 frequency domain function of Fig 3.4c

 frequency domain function of single square pulse

Spectrum & Bandwidth

Data Rate and Bandwidth any transmission system has a limited band of frequencies this limits the data rate that can be carried on the transmission medium limiting bandwidth creates distortions most energy in first few components square waves have infinite components and hence an infinite bandwidth There is a direct relationship between data rate and bandwidth.

Analog and Digital Data Transmission

 data  entities that convey information  signals  electric or electromagnetic representations of data  signaling  physically propagates along a medium  transmission  communication of data by propagation and processing of signals

Acoustic Spectrum (Analog)

IRA Digital Data Examples: Text Character strings

Advantages & Disadvantages of Digital Signals

Audio Signals

 frequency range of typical speech is 100Hz-7kHz  easily converted into electromagnetic signals  varying volume converted to varying voltage  can limit frequency range for voice channel to 300-3400Hz

Analog Signals

Digital Signals

Analog and Digital Transmission

Transmission Impairments

 signal received may differ from signal transmitted causing:   analog - degradation of signal quality digital - bit errors  most significant impairments are    attenuation and attenuation distortion delay distortion noise

Equalize attenuation across the band of frequencies used by using loading coils or amplifiers.

Received signal strength must be: •strong enough to be detected •sufficiently higher than noise to be received without error Strength can be increased using amplifiers or repeaters.

ATTENUATION



signal strength falls off with distance over any transmission medium



varies with frequency

Delay Distortion

 occurs because propagation velocity of a signal through a guided medium varies with frequency  various frequency components arrive at different times resulting in phase shifts between the frequencies  particularly critical for digital data since parts of one bit spill over into others causing intersymbol interference

Noise

unwanted signals inserted between transmitter and receiver is the major limiting factor in communications system performance

Categories of Noise Intermodulation noise

• produced by nonlinearities in the transmitter, receiver, and/or intervening transmission medium • effect is to produce signals at a frequency that is the sum or difference of the two original frequencies

Categories of Noise Crosstalk:

  a signal from one line is picked up by another can occur by electrical coupling between nearby twisted pairs or when microwave antennas pick up unwanted signals

Impulse Noise:

    caused by external electromagnetic interferences noncontinuous, consisting of irregular pulses or spikes short duration and high amplitude minor annoyance for analog signals but a major source of error in digital data

Channel Capacity Maximum rate at which data can be transmitted over a given communications channel under given conditions data rate bandwidth noise error rate in bits per second in cycles per second or Hertz average noise level over path rate of corrupted bits limitations due to physical properties main constraint on achieving efficiency is noise

Nyquist Bandwidth

In the case of a channel that is noise free:  if rate of signal transmission is

2B

then can carry signal with frequencies no greater than

B

 given bandwidth B, highest signal rate is 2B  for binary signals,

2B

bps needs bandwidth

B

Hz  can increase rate by using

M

signal levels  Nyquist Formula is:

C

= 2

B

log 2

M

 data rate can be increased by increasing signals  however this increases burden on receiver  noise & other impairments limit the value of

M

Shannon Capacity Formula

 considering the relation of data rate, noise and error rate:  faster data rate shortens each bit so bursts of noise corrupts more bits  given noise level, higher rates mean higher errors  Shannon developed formula relating these to signal to noise ratio (in decibels)  SNR db = 10 log 10 (signal/noise)  capacity 

C

=

B

log 2 (1+SNR) theoretical maximum capacity  get much lower rates in practice

Classifications of Transmission Media

 Transmission Medium  Physical path between transmitter and receiver  Guided Media  Waves are guided along a solid medium  E.g., copper twisted pair, copper coaxial cable, optical fiber  Unguided Media   Provides means of transmission but does not guide electromagnetic signals Usually referred to as wireless transmission  E.g., atmosphere, outer space

Unguided Media

 Transmission and reception are achieved by means of an antenna  Configurations for wireless transmission  Directional  Omnidirectional

General Frequency Ranges

 Microwave frequency range  1 GHz to 40 GHz    Directional beams possible Suitable for point-to-point transmission Used for satellite communications  Radio frequency range  30 MHz to 1 GHz  Suitable for omnidirectional applications  Infrared frequency range  Roughly, 3x10 11 to 2x10 14 Hz  Useful in local point-to-point multipoint applications within confined areas

Terrestrial Microwave

 Description of common microwave antenna     Parabolic "dish", 3 m in diameter Fixed rigidly and focuses a narrow beam Achieves line-of-sight transmission to receiving antenna Located at substantial heights above ground level  Applications   Long haul telecommunications service Short point-to-point links between buildings

Satellite Microwave

 Description of communication satellite  Microwave relay station   Used to link two or more ground-based microwave transmitter/receivers Receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink)  Applications  Television distribution   Long-distance telephone transmission Private business networks

Broadcast Radio

 Description of broadcast radio antennas    Omnidirectional Antennas not required to be dish-shaped Antennas need not be rigidly mounted to a precise alignment  Applications  Broadcast radio • • VHF and part of the UHF band; 30 MHZ to 1GHz Covers FM radio and UHF and VHF television

Multiplexing

 Capacity of transmission medium usually exceeds capacity required for transmission of a single signal  Multiplexing - carrying multiple signals on a single medium  More efficient use of transmission medium

Multiplexing

Reasons for Widespread Use of Multiplexing

 Cost per kbps of transmission facility declines with an increase in the data rate  Cost of transmission and receiving equipment declines with increased data rate  Most individual data communicating devices require relatively modest data rate support

Multiplexing Techniques

 Frequency-division multiplexing (FDM)  Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal  Time-division multiplexing (TDM)  Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal

Frequency-division Multiplexing

Time-division Multiplexing

Summary

 transmission concepts and terminology  guided/unguided media  frequency, spectrum and bandwidth  analog vs. digital signals  data rate and bandwidth relationship  transmission impairments  attenuation/delay distortion/noise  channel capacity  Nyquist/Shannon